1. Field of the Invention
This invention relates to a developing method for developing a latent image by the use of a developer and an apparatus therefor, and more particularly to a developing method using a one-component developer, especially a developing method which enables obtainment of fogless visible images excellent in sharpness and tone reproduction, and an apparatus therefor.
2. Description of the Prior Art
Various types of developing method using a one-component developer are heretofore known such as the powder cloud method which uses toner particles in cloud condition, the contact developing method in which a uniform toner layer formed on a toner supporting member comprising a web or a sheet is brought into contact with an electrostatic image bearing surface to effect development, and the magnedry method which uses a conductive magnetic toner formed into a magnetic brush which is brought into contact with the electrostatic image bearing surface to effect development.
Among the above-described various developing methods using one-component developer, the powder cloud method, the contact developing method and the magnedry method are such that the toner contacts both the image area (the area to which the toner should adhere) and the non-image area (the background area to which the toner should not adhere) and therefore, the toner more or less adheres to the non-image area as well, thus unavoidably creating the so-called fog.
To avoid such fog or background toner deposition, there has been proposed the transfer development with space between toner donor and image bearing member in which a toner layer and an electrostatic image bearing surface are disposed in opposed relationship with a space gap therebetween in a developing process so that the toner is caused to fly to the image area by the electrostatic field thereof and the toner does not contact the non-image area. Such development is disclosed, for example, in U.S. Pat. Nos. 2,803,177; 2,758,525; 2,838,997; 2,839,400; 2,862,816; 2,996,400; 3,232,190 and 3,703,157. This development is a highly effective method in preventing the fog. Nevertheless, the visible image obtained by this method generally suffers from the following disadvantages because it utilizes the flight of the toner resulting from the electric field of the electrostatic image during the development.
A first disadvantage is the problem that the sharpness of the image is reduced at the edges of the image. The state of the electric field of the electrostatic image at the edge thereof is such that if an electrically conductive member is used as the developer supporting member, the electric lines of force which emanate from the image area reach the toner supporting member so that the toner particles fly along these electric lines of force and adhere to the surface of the photosensitive medium, thus effecting development in the vicinity of center of the image area. At the edges of the image area, however, the electric lines of force do not reach the toner supporting member due to the charge induced at the non-image area and therefore, the adherence of the flying toner particles is very unreliable and some of such toner particles barely adhere while some of the toner particles do not adhere. Thus, the resultant image is an unclear one lacking sharpness at the edges of the image area, and line images, when developed, give an impression of having become thinner than the original lines.
To avoid this in the above-described toner transfer development, the clearance between the electrostatic image bearing surface and the developer supporting member surface must be sufficiently small (e.g. smaller than 100.mu.) and actually, accidents such as pressure contact of the developer and mixed foreign substances are liable to occur between the two surfaces. Also, maintaining such a fine clearance often involves difficulties in designing of the apparatus.
A second problem is that images obtained by the above-described toner transfer development usually back toner reproducibility. In the toner transfer development, the toner does not fly until the toner overcomes the binding power to the toner supporting member by the electric field of the electrostatic image. This power which binds the toner to the toner supporting member is the resultant force of the Van der Waals force between the toner and the toner supporting member, the force of adherence among the toner particles, and the reflection force between the toner and the toner supporting member resulting from the toner being charged. Therefore, flight of the toner takes place only when the potential of the electrostatic image has become greater than a predetermined value (hereinafter referred to as the transition threshold value of the toner) and the electric field resulting therefrom has exceeded the aforementioned binding force of the toner, whereby adherence of the toner to the electrostatic image bearing surface takes place. But the binding power of the toner to the supporting member differs in value from particle to particle or by the particle diameter of the toner even if the toner has been manufactured or prepared in accordance with a predetermined prescription, and therefore, it is considered to be distributed narrowly around a substantially constant value and correspondingly, the threshold value of the electrostatic image surface potential at which the flight of toner takes place also seems to be distributed narrowly around a certain constant value. Such presence of the threshold value during the flight of the toner from the supporting member causes adherence of the toner to that part of the image area which has a surface potential exceeding such threshold value, but causes little or no toner to adhere to that part of the image area which has a surface potential lower than the threshold value, with a result that there are only provided images which lack the tone gradation having steep .gamma. (the gradient of the characteristic curve of the image density with respect to the electrostatic image potential).
In view of such problems, a developing device in which a pulse bias of very high frequency is introduced across an air gap to ensure movement of charged toner particles flying through the air gap, whereby the charged toner particles are made more readily available to the charged image is disclosed in U.S. Pat. Nos. 3,866,574; 3,890,929 and 3,893,418.
Such high frequency pulse bias developing device may be said to be a developing system suitable for the line copying in that a pulse bias of several KHz or higher is applied in the clearance between the toner donor member and the image retaining member to improve the vibratory characteristic of the toner and prevent the toner from reaching the non-image area in any pulse bias phase but cause the toner to transit only to the image area, thereby preventing fogging of the non-image area. However, the aforementioned U.S. Pat. No. 3,893,418 states that a very high frequency (18 KHz-22 KHz) is used for the applied pulse voltage in order to make the device suitable for the reproduction of tone gradation of the image.
U.S. Pat. No. 3,346,475 discloses a method which comprises immersing two electrodes in insulating liquid contained in a dielectrophoretic cell and applying thereto an AC voltage of very low frequency (lower than about 6 Hz) to thereby effect the development of a pattern corresponding to the conductivity variance.
Further, U.S. Pat. No. 4,014,291 discloses a method in which dry, one component magnetic toner on the non-magnetic, non-conductive transfer cylinder which encloses a rotating cylindrical magnet is transferred to the deposit zone to develop an electrostatic latent image on coated paper, but this patent does not suggest that a bias is applied for the above-described purpose.
Further method for the image development is described in pending U.S. Patent applications Ser. No. 58,434 and No. 58,435 of the same assignee-to-be as that of the present application.
Thus, in order to obtain a toner image of good quality, the space gap between the electrostatic image holding member and the developer carrying member should be as close as possible at the developing section, e.g., 100.mu. to 200.mu.. By so doing, satisfactory polarity effect would appear, whereby an electric field due to the electrostatic latent image truthfully reaches the developer layer on the developer carrying member and good toner image can be obtained. This means that, since the developer layer on the developer carrying member should be extremely thin so as not to cause the developer to eventually contact the non-image portion.
Uniform application of the developer on the developer carrying member in such thin thickness (e.g., approx. 60.mu. to 150.mu.) has been difficult by the above-described conventional methods. Also, sure and uniform charging of each and every particle of the electrically insulative toner on the developer carrying member makes it necessary to reduce thickness of the toner layer on the developer carrying member to the thinnest possible extent, even when the frictional charging method, for example, is adopted for charging the toner on the developer carrying member. To attain this purpose, the developer is desired to have good fluidity and be readily chargeable.
More important in this developing method are that each and every toner particle is surely charged in an intended polarity, that each and every toner particle has good separability, and that the toner particles are in a state of being readily spattered in accordance with the electrostatic latent image field. These are the requisite conditions for obtaining a good quality image.
Furthermore, it is necessary that the developer which has adhered onto a part of the non-image portion due to the abovementioned alternating field be readily removed from the surface of the electrostatic image holding member by a subsequently applied opposite field. Without this condition being fulfilled, there would occur fogging phenomenon on the non-image portion. It is also desired that the toner particles are surely charged in an intended polarity.
As stated in the foregoing, and, particularly, in these developing methods, the developer layer should be coated on the developer carrying member to the thinnest possible extent. It is also desired that the toner particles constituting the developer be surely charged in an intended polarity, have good separability and high fluidity without being coagulated, and be easily spattered by the electric field.